1. Technical Field of the Invention
The present invention generally relates to catalysts and processes for improving catalyst performance in hydrocarbon processing—e.g., catalytic partial oxidation and catalytic oxidative dehydrogenation. More particularly, the present invention pertains to hydrocarbon processing catalysts and processes in which catalyst life is extended by reducing the formation of carbon deposits (“coking”) on the catalyst surface. Even more particularly, the present invention is directed to sulfided catalysts and processes utilizing sulfided catalysts that provide improved catalyst performance and life through reduced catalyst susceptibility to coking.
2. Description of Related Art
There is currently a significant interest in various types of hydrocarbon processing reactions designed to provide high value products from lower value reactants. Such reactions refer to any chemical process using hydrocarbons as a feedstock. One example of a hydrocarbon processing reaction involves the conversion of lower molecular weight gaseous hydrocarbons to higher molecular weight liquid hydrocarbons. Many refineries possess an abundant supply of relatively low value gaseous alkanes—i.e., such as methane and ethane—and few commercially-viable means of converting them to more valuable liquid alkanes. Moreover, vast reserves of methane, the main component of natural gas, are available in many areas of the world. There is great incentive to exploit these natural gas formations because natural gas is predicted to outlast oil reserves by a significant margin; however, most natural gas formations are situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage of gaseous hydrocarbons make their use economically unattractive relative to liquid hydrocarbons. Consequently, there is considerable interest in techniques for converting methane and other gaseous hydrocarbons to their heavier liquid hydrocarbon homologs both at refineries and especially at natural gas formations.
The conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is partially oxidized to produce a mixture of carbon monoxide and hydrogen known as synthesis gas or syngas. Syngas generation from methane typically takes place by one of three techniques: partial oxidation [1], steam reforming [2], and dry reforming [3]:CH4+1/2O2→CO+2H2  [1]CH4+H2O→CO+3H2  [2]CH4+CO2→2CO+2H2  [3].The syngas product in each case is a mixture of carbon monoxide and molecular hydrogen, generally having a hydrogen to carbon monoxide molar ratio in the range of 1:5 to 5:1, and may contain other gases such as carbon dioxide.
In a second step, the syngas is converted to liquid and solid hydrocarbons using the Fischer-Tropsch process. This method allows the conversion of synthesis gas into liquid hydrocarbon fuels and solid hydrocarbon waxes. In addition, the Fischer-Tropsch process allows the synthesis of alcohols (e.g., methanol) and olefins (e.g., ethylene). High molecular weight waxes provide an ideal feedstock for hydrocracking, a feedstock for conversion to high quality jet fuel and superior high octane value diesel fuel blending components.
Another category of hydrocarbon processing involves the oxidative dehydrogenation of hydrocarbons to give the corresponding alkene, such as the conversion of ethane to ethylene [4]:C2H6+1/2O2→C2H4+H2O  [4].Unsaturated hydrocarbons, such as the alkenes produced from oxidative dehydrogenation, possess industrial importance owing to their use as feedstocks in producing various commercially useful materials such as detergents, high octane gasolines, pharmaceutical products, plastics, synthetic rubbers and viscosity additives.
Catalysis plays a central role in syngas production, the Fischer-Tropsch reaction, oxidative dehydrogenation and many other hydrocarbon processing techniques. Each of these methods share a common attribute: successful commercial scale operation for catalytic hydrocarbon processing depends upon high hydrocarbon feedstock conversion at high throughput and with high selectivity for the desired reaction products. In each case, the yields and selectivities of catalytic hydrocarbon processing are controlled by several factors. One of the most important of these factors is the choice of catalyst composition, which significantly affects not only the yields and product distributions but also the overall economics of the process. Unfortunately, few catalysts offer both the performance and cost necessary for large-scale industrial use.
From an economic standpoint, desirable conditions for hydrocarbon processing include elevated pressures and temperatures, which translate into increased yields and throughput. Typically, though, hydrocarbon processing at elevated pressures is problematic because of shifts in equilibrium, undesirable secondary reactions and catalyst instability. Furthermore, the high operating temperatures desirable in hydrocarbon processing frequently cause catalyst instability and reduce catalyst life. Consequently, despite a variety of advances, sufficiently stable and long-lived catalysts capable of conversion rates that are attractive for large scale industrial use remain elusive. Accordingly, a continuing need exists for better hydrocarbon processing techniques and catalysts. In such improved processes the catalysts would be stable at high temperatures. They would also retain a high level of hydrocarbon conversion activity and selectivity under conditions of high gas space velocity and elevated pressures for long periods of time on-stream.
One primary cause of shortened catalyst life and reduced stability is coking, which is the formation of carbon deposits (“coke”) on the catalyst. Coking causes catalyst deactivation, thereby severely reducing catalyst performance. Substantial effort in this field has been devoted to the development of commercially-viable catalysts with reduced susceptibility to coke formation. Several techniques for minimizing coking have been developed. One technique is to add steam into the feed mixture. This technique, however, entails a number of disadvantages that render it unsatisfactory as a solution to the coking problem. Another technique for reducing coke formation is the use of noble metals, which are generally less susceptible to coking than the more widely used, less expensive, catalysts. Unfortunately, though, the noble metals are scarce and expensive. Furthermore, they are not immune to coking. No satisfactory solution to the coking problem currently exists.
Sulfur is traditionally viewed as a deleterious and undesirable contaminant in hydrocarbon processing reactions. This is true because many of the catalysts that are conventionally used in hydrocarbon processing regimes are believed to be poisoned by the presence of sulfur. Sulfur poisoning is of particular interest because many natural gas formations contain hydrogen sulfide (H2S) in concentrations ranging from trace amounts up to about forty percent by volume. Consequently, catalytic hydrocarbon processing techniques frequently employ a preliminary sulfur removal step.
Despite its reputation as a catalyst poison, sulfur has been employed in catalytic hydrocarbon processing. For example, sulfur-induced catalyst deactivation has been used to improve process selectivity through the selective deactivation of unwanted by-product reactions: because the deactivation of hydrocarbon catalysts with respect to unwanted side reactions can exceed deactivation of the desired process, deliberate catalyst sulfiding can increase catalyst selectivity at the expense of catalyst activity. Under appropriate circumstances, this trade-off of improved selectivity for decreased activity can prove advantageous.
Recently, it has been suggested that sulfur may have some utility in reducing the generation of undesirable nitrogen by-products in natural gas refining. U.S. Pat. No. 5,720,901 (“the '901 patent”) describes a process for the catalytic partial oxidation of hydrocarbons in which nitrogen is present in the hydrocarbon feed mixture. According to the '901 patent, an organic or inorganic sulfur-containing compound is present in the feed mixture in a sufficient concentration (i.e., 0.05 to 100 ppm) to reduce the presence of nitrogen by-products, particularly ammonia and hydrogen cyanide, in the products of the catalytic partial oxidation process. It is suggested that hydrocarbon feedstocks used directly from naturally occurring reservoirs in which the sulfur content is significantly above the stated low levels be subjected to a partial sulfur removal treatment before being employed in that process. A sulfur removal step is applied to the product stream if the carbon monoxide and/or hydrogen products of the process are to be utilized in applications that are sensitive to the presence of sulfur, such as Fischer-Tropsch synthesis. Generally speaking, though, sulfur is believed to be a catalyst poison that is deleterious in catalytic hydrocarbon processing.
What is needed is a hydrocarbon processing catalyst and a method of using a hydrocarbon processing catalyst that suppresses the formation of coke and thereby promotes catalyst activity and extends catalyst life. This catalyst and method must be able to achieve a high conversion of the hydrocarbon feedstock with high throughput and product selectivity. Not only is the choice of the catalyst's chemical composition important, the physical structure of the catalyst and catalyst support structures must possess mechanical strength and porosity, in order to function under operating conditions of high pressure and high flow rate of the reactant and product gases.